This invention relates generally to a communication system and more particularly, to a system for communicating commands and data between a central station and a plurality of electricity meters.
Typically, utility companies have service personnel visit customer sites to read electricity meters or other meters (utility meters) at the sites to determine the amount of electrical energy consumed for billing purposes. This meter reading technique is cumbersome and time consuming particularly when there are many consumer sites over a large geographic area.
As a result of these difficulties involved in reading utility meters, many systems have been proposed for automated meter reading (AMR) operations. The AMR systems enable utility companies to more easily obtain energy consumption data from many different sites without having service personnel visit each consumer site. Some AMR systems use power line communications. Other AMR systems use radio frequency communications. Still other AMR systems use a hybrid approach that involves both power line communication and radio frequency communication. Typically, most AMR systems transmit data and commands between a central station and the utility meters.
In particular, some AMR communication systems implement radio frequency communications based on the unlicensed use of the 902-928 MHz ISM band. However, the ISM band is becoming crowded with users, and as a result, interference is becoming a problem. Additionally, fixed-network AMR systems using the high frequencies, such as the ISM band, encounter problems relating to the placement of concentrators because the high frequency signals can be transmitted over relatively small distances. As a result, these high frequency communication networks may require the placement of many concentrators. Thus, the implementation of these AMR systems becomes less economically feasible. As such, there is a desire to provide AMR systems that have the flexibility of radio frequency communications and/or a hybrid power line/radio frequency communications and that eliminates problems associated with communication in the ISM band, such as, for example, placement of many concentrators.
The commercial amplitude modulation (AM) band is an alternative candidate for low data rate, unlicensed transmissions. Unlicensed transmitters in the AM band (510 kHz to 1705 kHz) are limited to 100 mW input to the final transmitter stage and transmit antenna length of less than three meters. Commercial AM stations are spaced 10 kHz apart with no guard bands, but there are typically many unused station frequencies in a given region, and many AM stations transmit signals whose bandwidth is less than that of the allocated band. These facts suggest that an interference-free part of the AM band could be found for data transmission at any given time. It should be noted, however, that interference from AM stations will vary with the time of day and with atmospheric conditions.
Another aspect of radio communication which should be considered is multipath signal cancellation. The wavelengths in the AM band range from about 600 feet to about 2000 feet. Propagation path length differences equal to some small multiple of the wavelength, plus or minus half a wavelength, will cause some degree of cancellation of the signal at the receiver. This effect can be mitigated by increasing the transmitted power or, alternatively, the bandwidth of the transmitted signal. One well-known way of increasing the signal""s bandwidth is through the use of multi-tone modulation (MTM). Further, MTM signals can be received adaptively in such a way as to maximize the signal-to-noise ration of the reception. This technique has been called adaptive maximal ratio combining.
The problem of using long-range RF to gather utility meter-reading and other data from local collection points is addressed by unlicensed transmission in the AM radio band. The problem of interference from primary users of that band is addressed by the use of multi-tone modulation, which allows the receiver to selectively use for demodulation those parts of the transmission that lie in clear portions of the band. This enables a one-way transmission in the AM band without prior knowledge on the part of the transmitter as to where in the band interfering transmissions lie. For a two-way transmission in the AM band, it provides a signaling scheme that can be dynamically optimized to suit the observed state of the channel. This modulation also overcomes any multipath signal cancellation that might attenuate a transmission comprised of a single narrowband signal.
In one representative embodiment, a communication system for communicating utility data from a utility distribution network is provided. The communication system comprises a transmitter for transmitting the utility data. The transmitter comprises a digital signal processor that modulates the utility data into a plurality of carrier frequencies. These frequencies are spaced at an integer multiple of the spacing of AM radio stations, and they fall midway between the center frequencies of the standard AM radio station bands. A digital-to-analog converter is connected to the digital signal processor and converts the modulated utility data into an analog signal. A wide band radio frequency (RF) modulator is connected to the digital-to-analog converter and receives the analog signal. The wide band RF modulator modulates the analog signal in the AM frequency band, and the transmitter transmits the AM modulated analog signal in the AM frequency band. In the representative embodiment, the communication system also comprises a receiver. The receiver comprises a wide band radio frequency (RF) demodulator that receives the transmitted AM modulated analog signal. The wide band RF demodulator demodulates the AM modulated analog signal into a received analog signal. An analog-to-digital converter is connected to the wide band RF demodulator and converts the received analog signal to received digital data. A digital signal processor is connected to the analog-to-digital converter and receives the received digital data. The digital signal processor constructs the received digital data forming received utility data.